digital inputs: Network interface on an RJ45 connector – Acts as a UPnPTM renderer in Asynchronous mode, streaming digital music from a NAS or local computer over a standard Ethernet network, decoding all major lossless formats including FLAC, WAV & AIFF at up to 24 bit 384kS/s native sample rate, plus DSD/64 & DSD/128 in DFF/DSF format
Other supported formats include WMA, ALAC, MP3, AAC & OGG.
Some formats are limited to lower sample rates. Accepts data streamed from an iPod, iPhone or iPad via Apple AirPlay, 44.1 or 48kS/s only. Network Loop Out connector on a second RJ45 connector. USB 2.0 interface on a B-type connector operating in Asynchronous mode, will accept up to 24 bit PCM at up to 384kS/s plus DSD/64 & DSD/128 in DoP format. Operates in Class 1 or 2 mode.
USB-on-the-go interface on type A connector operating in Asynchronous mode, streams digital music from a flash drive at up to 24 bit 384kS/s plus DSD/64 or DSD/128.
2x AES/EBU on 3-pin female XLR connectors.
Each will accept PCM at up to 24 bit 192kS/s or DSD/64 in DoP format. Used as a Dual AES pair, it will accept PCM at up to 384kS/s, DSD/64 & DSD/128 in DoP format or dCS-encrypted DSD.
2x SPDIF on 1x RCA Phono and 1x BNC connectors. Each will accept PCM at up to 24 bit 192kS/s or DSD/64 in DoP format. 1x SPDIF optical on a Toslink connector will accept PCM at up to 24 bit 96kS/s.
analogue outputs: Output levels: 2V or 6V rms for full-scale input, set in the menu. Balanced outputs: 1 stereo pair on 2x 3-pin XLR male connectors (pin 2 = hot, pin 3 = cold). These outputs are electronically balanced and floating.
Output impedance is 3Ω, maximum load is 600Ω (10k-100kΩ is recommended).
Unbalanced outputs: 1 stereo pair on 2x RCA phono connectors.

Wordclock I/O: 2x Word Clock Inputs on 2x BNC connectors, accept standard word clock at 44.1, 48, 88.2, 96, 176.4 or 192kHz.
The data rate can be the same as the clock rate or an exact multiple of the clock rate. Sensitive to TTL levels. Word Clock Output on 1x BNC connector. In Master mode, a TTL-compatible word clock appears on this output.

local control: dCS Rossini App for Unit Configuration and Music Playback. RS232 interface (controlled by a 3rd party automation system). IR remote control – dCS premium remote control available as an optional extra.

power supply: Factory set to either 100, 115/120, 220 or 230/240V AC, 50-60Hz.

I like to know if NAS hard drive is necessary for having a good sound or any ext-hard connected via USB or Thunderbolt directly to computer is okay? I am going to install Roon on my Mac Air and looking for best way to keep my music file.

If your question is with respect to the Rossini, IMHO the differences between a NAS and a USB connection directly from your computer to the DAC will be minor. Better--though more costly -- ways to reduce noise in your system include upgrading your cables; your pre-amp, adding a power conditioner, or best, adding the Rossini clock.